International Consolidated Airlines Group SA IAG.MC +1.73% could as soon as this week place an order for the new Airbus A350 long-range jetliner for its British Airways unit, according to two people familiar with the talks.

If completed, the deal would be a significant win for the Airbus unit of European Aeronautic Defence & Space Co. EAD.FR +0.09% over rival Boeing Co., BA -0.70% which has been trying to persuade IAG to buy a proposed update of its 777 model, dubbed the 777X, intended to compete with the A350.

Boeing hasn't started formally offering the 777X yet—the company needs approval to launch the model from its board. A decision is expected as early as this month. But the Chicago company has provided extensive detail about the updated jet to prospective clients, and has made offers conditional on board approval to sell the plane to airlines and lessors including IAG, Japan Airlines Co., 9201.TO +0.82% Deutsche Lufthansa AG, LHA.XE -0.68% Air Lease Corp. AL -1.47% and Emirates Airline, according to industry officials. Boeing aims to deliver the first new 777 model to customers by late 2019 or early 2020.

"We continue to see IAG as an important customer, and look forward to working with IAG to meet its fleet needs," said Karen Crabtree, Boeing spokeswoman. She said Boeing has been "developing options to improve on the 777's popularity" and that customers are "happy with the airplane design."

Even if IAG orders the A350 for British Airways, Boeing is "not out of the running" for a later order for the 777Xs, said one of the people familiar with the carrier's plans. But another person said that if IAG orders A350s, it wouldn't receive preferential terms on Boeing orders.

Early buyers of a new jetliner model, known as launch customers, generally receive preferential terms and big discounts, potentially exceeding 50% off catalog prices.

Boeing's product development unit was scheduled to meet on Monday with top executives and the company's senior advisory group, which is made up of retired top engineers that designed the company's earlier jetliners. The meeting is an important step in the process of preparing the jet's business model ahead of seeking the board's approval, said two people familiar with the meeting.

British Airways currently has 52 of the existing versions of the long-range 777 that is among Boeing's best-selling planes. The carrier currently flies no Airbus long-range, twin-aisle jetliners but is due soon to receive its first Airbus A380 superjumbo, which it ordered alongside Boeing's 787 Dreamliner in 2007.

The exact number of planes in IAG's A350 order, and its value, are still under negotiation, according to the people familiar with the talks. IAG's talks with Airbus focus on the A350-1000 model, the largest of three A350 versions Airbus is developing and is due in 2017, the people said. The order could also include a smaller, longer-range version, the A350-900, one of these people said.

The purchase of the largest A350 model is likely designed to replace the company's aging fleet of 747-400 aircraft, which were delivered from 1989 to 1999.

An IAG spokeswoman declined to comment on possible orders.

Japan Airlines, which has long operated an all-Boeing fleet, also is weighing a possible order for the A350, one person familiar with the situation said.

"When we look to introduce new planes we weigh all of the options available to us whether it's from Airbus or Boeing. But JAL presently has no specific plans to purchase the Airbus A350," said a JAL spokesman.

IAG was created in 2011 by the merger of British Airways with Spain's Iberia. Iberia's fleet is all Airbus planes. British Airways' short and medium-haul European fleet is all Airbus models and its twin-aisle long-range jets have been Boeing jets for nearly a decade.